Hi,
Thank you for posting your query here.
In order to answer your first query "Is it possible to add a user and an extra license without giving access to the administrators files" -> You may add as many licenses for Team users and each user gets their individual share of Cloud storage and any user do not have access to view the files stored by other user or Administrator in their respective cloud storage. Unless the administrator or other user specifically Shares the files or Folders, the data remains restricted only to the same user.
To address your query regarding having a user manage the contracts for Clients and saving the files on Cloud Storage, this option is not possible as the User who is managing the contracts will by default be able to view the files as well & if you do not provide Collaboration Access or share the files with the user, he/ she will not be able to view and Manage the contracts.
You may as well refer to file Collaboration help article that may be of further help : https://helpx.adobe.com/in/creative-cloud/help/collaboration-faq.html
Thank you.